1. Field of the Invention
The present invention relates to vacuum pumps used in semiconductor manufacturing apparatus, and more particularly, the present invention relates to the structure of a vacuum pump for preventing a brittle fracture of a fastening bolt that connects the vacuum pump and a process chamber, which is caused by a damaging torque.
2. Description of the Related Art
In a process such as dry etching, chemical vapor deposition (CVD), or the like performed in a high-vacuum process chamber in semiconductor manufacturing step, a vacuum pump such as a turbo-molecular pump is used for producing a high vacuum in the process chamber by exhausting gas from the process chamber
FIG. 1 illustrates the basic structure of such a vacuum pump. As shown in FIG. 1, the vacuum pump has a cylindrical pump case 1 having a bottom, and the pump case 1 has an opening at the top portion thereof serving as a gas suction port 2 and an exhaust pipe, at a lower part of the cylindrical surface thereof, serving as a gas exhaust port 3.
The bottom portion of the casing 1 is closed by an end plate 4, and a stator column 5 stands upright at a center portion of the internal bottom surface of the end plate 4.
A rotor shaft 7 is rotatably supported by an upper ball bearing 6 and a lower ball bearing 6 at the center of the stator column 5.
A driving motor 8 is arranged inside the stator column 5. The driving motor 8 has a structure in which a stator element 8a is disposed on the rotor shaft 7, and it is structured such that the rotor shaft 7 is rotated about the shaft.
A rotor 9, which covers the outer circumference of the stator column 5 and is formed in a section-shape, is connected to the upper portion protrusion end from the stator column 5 of the rotor shaft 7.
A plurality of rotor blades 10 are disposed and fixed to the upper part of the circumferential outer surface of the rotor 9, while a plurality of stator blades 11 are alternately disposed with respect to the rotor blades 10 and are fixed to each other inside the pump case 1 via ring spacers 11a. 
The pump case 1 has a threaded stator 12 disposed and fixed under the blades 10 and 11 and around the rotor 9. The threaded stator 12 is formed in a tapered cylindrical shape so as to surround the outer circumferential surface of the lower part of the rotor 9 and its inner surface has a tapered shape, the inner surface of which has a diameter that gradually decrease downwardly. Also, the threaded stator 12 has thread grooves formed on the tapered inner surface thereof.
A flange 1a is formed along the circumferential uppermost portion of the pump case 1. The flange 1a is fitted on the peripheral end of an opening portion of the lower surface side of a process chamber (hereinafter, referred to as xe2x80x9cchamberxe2x80x9d) 14 and a plurality of fastening bolts 15, which penetrate the flange 1a, are screwed in and fixed to the chamber 14, so that the pump case 1 is connected to the chamber 14.
Next, the operation of the foregoing vacuum pump will be described. In this vacuum pump, firstly, an auxiliary pump (not shown) connected to the gas exhaust port 3 is activated so as to evacuate the chamber 14 to a certain vacuum level. Then, the driving motor 8 is operated so as to rotate the rotor shaft 7, the rotor 9 connected to the rotor shaft 7, and the rotor blades 10 also connected to the rotor shaft 7 are rotated at high speed.
When the rotor blade 10 rotates at high speed, at the uppermost stage the rotor blade 10 imparts a downwards momentum to the gas molecules entering through the gas suction port 2, and the gas molecules with this downward momentum are guided by the stator blades 11 to be transferred to the next lower rotor blade 10 side. By repeating this imparting of momentum to the gas molecules and transferring operation, the gas molecules are transferred from the gas suction port 2 to the inside of the threaded stator 12 provided on the lower portion side of the rotor 2 in order. The above-described operation of exhausting gas molecules is called a gas molecule exhausting operation performed by the interaction between the rotating rotor blades 10 and the stationary stator blades 11.
The gas molecules reaching the threaded stator 12 by the above-described gas molecule exhaust operation are compressed from an intermediate flow state to a viscous flow state, are transferred toward the gas exhaust port 3 by the interaction between the rotating rotor 9 and the thread grooves formed inside the threaded stator 12 and are eventually exhausted to the outside via the gas exhaust port 3 by the auxiliary pump (not shown).
Incidentally, as structural materials of the casing 1, the rotor 9, the rotor blade 10 and the stator blade 11 or the like, which compose the vacuum pump, light alloy, in particular, aluminum alloy is normally employed in many cases. This is because aluminum alloy is excellent in machining and can be precisely processed without difficulty. However, the hardness of aluminum alloy relatively low as compared with other materials and aluminum alloy may cause a creep fracture depending on the operating condition. Further, a brittle fracture may occur in operation mainly caused by a stress concentration at the lower part of the rotor 9.
If the brittle fracture occurs in the rotor 9 during a high speed rotation, some of the rotor blades 10 integrally formed with the circumferential outer surface of the rotor 9 crash into the ring spacers 11a disposed on the circumferential inner surface of the pump case 1. Since the ring spacers 11a have insufficient strength against this smashing force, the smashing force causes the ring spacers 11a to expand in the radial direction thereof. When a sufficient clearance is not provided between the ring spacers 11a and the circumferential inner surface of the pump case 1, the expanded ring spacers 11a come into contact with the circumferential inner surface of the pump case 1, thereby producing a large damaging torque which causes the whole pump case 1 to rotate, and accordingly, this damaging torque causes the chamber 14 to be broken or the torsional moment due to the damaging torque causes the bolts 15 fastening the pump case 1 to the-chamber 14 to be broken by shearing.
Since such a damaging torque causes the contact surface of the flange 1a of the pump case with the chamber 14 to act as a sliding surface and two very large forces to be instantaneously exerted on a portion, lying in the vicinity of the contact surface, of the bolt shaft of each bolt 15 in opposite directions, the bolt 15 is easily broken at the foregoing portion acting as a breaking surface, thereby leading to the above-described shearing breakage. Once the bolt 15 is broken, since its bolt shaft cannot be extracted from the corresponding hole of the chamber 14, the bolt shaft left in the chamber 14 must be removed by tapping. Also, replacing the damaged vacuum pump with a new one is troublesome.
The present invention is made so as to solve the above-described problems. It is an object of the present invention to provide a vacuum pump which prevents a chamber and fastening bolts, connecting the pump to the chamber, from being broken even when a damaging torque occurs caused by a trouble in the pump, and which can be quickly replaced with a new one.
To attain the above described object, a vacuum pump according to the present invention comprises a pump case including a gas suction port formed at an upper surface of the pump case and a gas exhaust port formed at a lower part of the cylindrical surface of the pump case; a rotor rotatably supported by a stator column via a rotor shaft, wherein the rotor is provided with a plurality of rotor blades fixed to the circumferential outer surface of the rotor and the stator column is disposed upright in the pump case; a plurality of stator blades fixed to the circumferential inner surface of the pump case, the rotor blades and the stator blades being alternately disposed; a driving motor disposed between the rotor shaft and the stator column; a plurality of bolts for connecting a flange to the circumferential bottom portion of a chamber, wherein the flange is formed along the circumferential top portion of the pump case; and a plurality of bolt insertion holes having stages which increase in size step by step toward the fixing surface of the chamber.
In the vacuum pump having the above-described structure according to the present invention, when the damaging torque is generated, the shearing force at the upper edge of each step caused by the damaging torque moves upwards step by step and does not concentrate on one specific upper edge, and the shock caused by the damaging torque is absorbed during this time period. As a result, the bolt shaft of the bolt merely undergoes a plastic deformation, thereby preventing the damaging torque from being transferred to the chamber so that the chamber is prevented from being damaged, and also preventing the bolt from being broken.
The vacuum pump according to the present invention may further comprise a buffer member disposed between the inner wall of the bolt insertion hole and the bolt shaft of the corresponding bolt. With this structure, the buffer effect of the elastically deformed buffer member prevents the damaging torque from being transferred to the chamber so that the chamber is prevented from being damaged, and also prevents the bolt from being broken.
The vacuum pump according to the present invention may have a structure in which the bolt insertion hole may have two steps having large and small diameters and the buffer member may be disposed between the bolt shaft and the large step portion close to the chamber.
Alternatively, the vacuum pump may further comprise a washer disposed between the bolt head and the flange, and has a structure in which the buffer member has an insertion hole for the bolt shaft to pass therethrough, and the bolt shaft and the upper part of the buffer member having an enlarged inner diameter have a gap therebetween.
Still alternatively, the vacuum pump may have a structure in which the bolt insertion hole has a tapered shape which increases in size toward the fixing surface of the chamber and the buffer member having a truncated cone shape is disposed between the bolt shaft and the bolt insertion hole.
A variety of devised shapes and structures of the buffer members disposed between the bolt shaft and the bolt insertion hole prevent the damaging torque from being transferred to the chamber so that the chamber may be prevented from being damaged, and also prevent the bolt from being broken.
In the vacuum pump according to the present invention, the bolt is preferably an extending bolt comprising a reduced-diameter portion between the bolt head and the male-threaded portion thereof and the diameter of the reduced-diameter portion is preferably smaller than the root diameter of the male-threaded portion.
In the vacuum pump according to the present invention, the extending bolt is preferably screwed into the chamber such that the top of the reduced-diameter portion enters the chamber by the length of one or two threads of the bolt.
In the vacuum pump according to the present invention, the buffer member may be composed of a rubber material.